Class
ActionController::API
<
Metal

API Controller is a lightweight version of
ActionController::Base, created for applications that
don't require all functionalities that a complete Rails controller
provides, allowing you to create controllers with just the features that
you need for API only applications.

An API Controller is different from a normal
controller in the sense that by default it doesn't include a number of
features that are usually required by browser access only: layouts and
templates rendering, cookies, sessions, flash, assets, and so on. This
makes the entire controller stack thinner, suitable for API applications. It doesn't mean you won't
have such features if you need them: they're all available for you to
include in your application, they're just not part of the default API controller stack.

Normally, ApplicationController is the only controller that
inherits from ActionController::API. All other controllers in
turn inherit from ApplicationController.

Request, response, and parameters objects all work the exact same way as
ActionController::Base.

Renders

The default API Controller stack includes all
renderers, which means you can use render :json and brothers
freely in your controllers. Keep in mind that templates are not going to be
rendered, so you need to ensure your controller is calling either
render or redirect_to in all actions, otherwise
it will return 204 No Content.

def show
post = Post.find(params[:id])
render json: post
end

Redirects

Redirects are used to move from one action to another. You can use the
redirect_to method in your controllers in the same way as in
ActionController::Base. For example:

def create
redirect_to root_url and return if not_authorized?
# do stuff here
end

Adding New Behavior

In some scenarios you may want to add back some functionality provided by
ActionController::Base that is not present by default in
ActionController::API, for instance MimeResponds.
This module gives you the respond_to method. Adding it is
quite simple, you just need to include the module in a specific controller
or in ApplicationController in case you want it available in
your entire application:

[
AbstractController::Rendering,
UrlFor,
Redirecting,
ApiRendering,
Renderers::All,
ConditionalGet,
BasicImplicitRender,
StrongParameters,
ForceSSL,
DataStreaming,
# Before callbacks should also be executed as early as possible, so
# also include them at the bottom.
AbstractController::Callbacks,
# Append rescue at the bottom to wrap as much as possible.
Rescue,
# Add instrumentations hooks at the bottom, to ensure they instrument
# all the methods properly.
Instrumentation,
# Params wrapper should come before instrumentation so they are
# properly showed in logs
ParamsWrapper
]